Various lithographic printing plates requiring no fountain solution for lithographic printing have been proposed. Among them, those comprising a support having formed thereon a photosensitive layer and an ink repellent layer in this order exhibit excellent performance. Materials of the ink repellent layer so far proposed include silicone rubber (e.g., JP-B-54-26923, JP-B-55-22781, JP-B-56-23150, JP-A-2-236550, the term "JP-B" as used herein means an "examined Japanese patent publication", and the term "JP-A" as used herein means an "unexamined published Japanese patent application") and fluorine resins (e.g., JP-A-58-215411, JP-A-2-103047). The silicone rubber is preferred for its ink repellency.
The photosensitive layer for use in positive lithographic printing plates comprises a photopolymerizable photosensitive composition which cures on exposure to light. Platemaking using this type of plates is generally carried out as follows. On exposure, the photosensitive layer on non-image area is cured and adhered firmly to the upper silicone rubber layer. In the subsequent development, a developer is prevented from penetrating into the exposed non-image area and from dissolving the photosensitive layer on that area thereby to form a non-image area composed of a silicon rubber layer. On the other hand, the unexposed image area is penetrated with a developer through the silicone rubber layer, and part of or the whole of the uncured photosensitive layer is dissolved with the developer. Thereafter, the silicone rubber layer on that area is removed by a physical force to form an image area.
Developing techniques for the lithographic printing plate requiring no fountain solution which can form an image as described above are disclosed in JP-B-56-23150, JP-A-57-13448, JP-A-59-146054 and JP-A-63-52145. However, since the developers used in these techniques mainly comprise an organic solvent, they are expensive and also need special care in handling because of their danger of ignition. To solve this problem, JP-A-1-159644 discloses use of a developer mainly comprising water. It is necessary that the photosensitive layer composition should contain a hydrophilic compound as a photopolymerizable monomer before such an aqueous developer can be employed. For example, monomers having a polyethylene oxide chain or a polypropylene oxide chain are disclosed in JP-A-4-174437, JP-A-4-338954, JP-A-5-94007, JP-A-5-224415, and JP-A-6-118629. A satisfactory image can be obtained by using the monomers proposed.
However, none of the conventional lithographic printing plates requiring no fountain solution which are developed with an aqueous developer satisfies all the requirements of sensitivity, tone reproducibility, press life, and scratch resistance sufficiently for practical use.